The present invention is directed to oscillators. More particularly, the invention provides an oscillator that can provide a periodic signal at a low frequency with high precision. Merely by way of example, the invention has been applied to burst-mode dimming control for cold-cathode fluorescent lamp (CCFL) backlight driver system. But it would be recognized that the invention has a much broader range of applicability. For example, the present invention can be applied to integrated circuit systems other than a CCFL backlight driver system. In another example, the present invention can be applied to devices other than integrated circuits.
The burst-mode dimming control technology has been widely used in cold-cathode fluorescent lamp (CCFL) backlight driver systems to control the brightness of the CCFL. In the burst-mode dimming control, a low-frequency oscillator often is needed. To implement a low-frequency oscillator, a variety of conventional techniques have been used to generate a periodic signal at a desired frequency.
For example, conventional relaxation oscillators or multivibrators are widely used as low-frequency oscillators in monolithic integrated circuit designs. Such relaxation oscillators may be R-C charge and discharge oscillators, constant-current charge and discharge oscillators, and/or emitter-coupled multivibrators. To achieve low oscillation frequency, off-chip resistors and off-chip capacitors are usually needed.
FIG. 1 is a simplified diagram showing a conventional low-frequency relaxation oscillator. The oscillator 100 operates by alternately charging and discharging an external timing capacitor Cext between two internally-set threshold voltage levels VH and VL. Such charging and discharging result in the generation of a periodic output clock signal LCLK, whose frequency is inversely proportional to the capacitance value of the timing capacitor.
FIG. 2 is a simplified conventional diagram showing waveforms that are generated by a low-frequency relaxation oscillator. For example, the low-frequency relaxation oscillator is the oscillator 100. In another example, the waveforms for VH, VL, Vramp, and LCLK each represent the signal voltage as a function of time.
As shown in FIGS. 1 and 2, one or more external resistors often are used to form a constant current source IC and a constant current sink ID. The current IC is used to charge the external timing capacitor Cext as follows:
                              T          ON                =                                            (                                                V                  H                                -                                  V                  L                                            )                        ×                          C              o                                            I            C                                              (                  Equation          ⁢                                          ⁢          1                )            
where TON is the charging time, and CO is the capacitance of the timing capacitor Cext. Also, the current ID is used to discharge the timing capacitor Cext as follows:
                              T          OFF                =                                            (                                                V                  H                                -                                  V                  L                                            )                        ×                          C              O                                            I            D                                              (                  Equation          ⁢                                          ⁢          2                )            
where TOFF is the discharging time. Hence the switching frequency FS is determined by the charging and discharging of the capacitor as follows:
                              F          S                =                              1                                          T                ON                            +                              T                OFF                                              =                                    1                                                (                                                            V                      H                                        -                                          V                      L                                                        )                                ⁢                                  C                  O                                                      ×                                                            I                  C                                ⁢                                  I                  D                                                                              I                  C                                +                                  I                  D                                                                                        (                  Equation          ⁢                                          ⁢          3                )            
But the conventional low-frequency oscillators often are costly and/or low in precision. Hence it is highly desirable to improve techniques for oscillators.